Ink-jet printing systems have been employed utilizing intermediate transfer surfaces, such as that described in U.S. Pat. No. 4,538,156 of Durkee et al. in which an intermediate transfer drum is employed with a printhead. A final receiving surface of paper is brought into contact with the intermediate transfer drum after the image has been placed thereon by the nozzles in the printhead. The image is then transferred to the final receiving surface.
U.S. Pat. No. 5,099,256 of Anderson describes an intermediate drum with a surface that receives ink droplets from a printhead. The intermediate drum surface is thermally conductive and formed from a suitable film-forming silicone polymer allegedly having a high surface energy and high degree of surface roughness to prevent movement of the ink droplets after receipt from the printhead nozzles. Other imaging patents, such as U.S. Pat. Nos. 4,731,647 and 4,833,530 of Kohsahi, describe a solvent that is deposited on colorant to dissolve the colorant and form a transferable drop to a recording medium. The colorants are deposited directly onto paper or plastic colorant transfer sheets. The transferable drops are then contact transferred to the final receiving surface medium, such as paper.
U.S. Pat. No. 4,673,303 of Sansone et al. describes an offset ink-jet postage printing method and apparatus in which an inking roll applies ink to the first region of a dye plate. A lubricating hydrophilic oil can be applied to the exterior surface of the printing drum or roll to facilitate the accurate transfer of the images from the drum or roll to the receiving surface.
The above-described processes do not achieve a complete image transfer from the intermediate transfer surface under normal printing conditions and, therefore, require a separate cleaning step to remove any residual ink from the intermediate receiving surface. Prior intermediate transfer surfaces also have not been renewable.
The prior processes are also limited in the degree of image quality that can be achieved on different types of final receiving surfaces or print media. Because the inks are fluids, they are subject to uncontrolled bleeding on porous media, such as paper, and uncontrolled spreading on transparency films or glossy coated papers.
The above-described problems are addressed by processes and apparatus described in co-pending U.S. patent application Ser. Nos. 07/981,646 and 07/981,677, both filed Nov. 25, 1992 and assigned to the assignee of this application. A transfer printer employing phase-change ink is described in which a liquid intermediate transfer surface is provided that receives a phase-change ink image on a drum. The image is then transferred from the drum with at least a portion of the intermediate transfer surface to a final receiving medium, such as paper.
In particular, the phase-change ink transfer printing process begins by first applying a thin liquid intermediate transfer surface to the drum. Then an ink-jet printhead deposits molten ink onto the drum where it solidifies and cools to about the temperature of the drum. After depositing the image, the print medium is heated by feeding it through a preheater and into a nip formed between the drum and an elastomeric transfer roller. As the drum turns, the heated print medium is pulled through the nip and is pressed against the deposited image, thereby transferring the ink to the print medium. When in the nip, heat from the print medium heats the ink, making it sufficiently soft and tacky to adhere to the print medium. When the print medium leaves the nip, stripper fingers peel it from the drum and direct it into a media exit path.
In practice, it has been determined that a transfer printing process should meet at least the following criteria to produce acceptable prints. To optimize image resolution, the transferred ink drops should spread out to cover a predetermined area, but not so much that image resolution is lost. The ink drops should not melt during the transfer process. To optimize printed image durability, the ink drops should be pressed into the paper with sufficient pressure to prevent their inadvertent removal by abrasion. Finally, image transfer conditions should be such that substantially all of the ink drops are transferred from the drum to the paper.
Unfortunately, the proper set of image transfer conditions are dependent on a complexly interrelated set of pressure, temperature, time, and ink parameters that have not been well understood, thereby preventing phase-change transfer printing from meeting its full potential for rapidly producing high-quality prints.
What is needed, therefore, is a phase-change transfer printing process and apparatus that addresses the problems and challenges of controlling the image transfer conditions to rapidly produce consistently high-quality prints on a wide range of print media.